This invention relates generally to a mode coupled tuning fork type quartz crystal resonator of the type used in electronic circuitry such as timepieces, more particularly, to a mounted mode coupled tuning fork type quartz crystal resonator which is isolated by the mounting to prevent loss of energy and reduction in oscillatory Q factor. If the lower part of the tuning fork base, that is, the portion excepting the two tuning fork arms, is made completely stationary, there will be no leakage of energy at this lower edge. However, to utilize this method, the displacements in the base portion of the resonator must be small and the mass of the supporting system must be greater than that of the resonator. Otherwise, a leakage of acoustic energy occurs. Another method to prevent the vibrations of the resonator from being induced into the mounting plug is by lengthening the support members which connect the plug to the base portion of the tuning fork. This can be beneficial even though displacements in the tuning fork base portion increase to some degree.
These methods are practically suitable to prevention of loss of energy in the flexural fundamental vibrational mode, but these methods are not applicable to isolating the flexural overtone frequency. Further, these methods are never used in a mode mode coupled tuning fork type quartz crystal resonator where displacements of the flexural overtone or the coupled vibrational mode are significantly large as compared to the displacements of the flexural fundamental vibration. When these methods have been applied to such vibrational modes, the characteristics of Q, crystal impedance and frequency changes due to aging have been less favorable. Especially for a mode coupled tuning fork type quartz crystal resonator, it is desirable that the acoustic leak of the torsional vibrational mode be completely eliminated.
What is needed is a mounted mode coupled tuning fork type quartz crystal resonator having good energy loss isolation in the fundamental flexural and torsional modes of vibration as well as in the overtone mode of flexural vibration.